1. Field of the Invention
The present invention relates to an electromagnetic wave absorber used for preventing the external leakage of unnecessary electromagnetic waves generated in communication equipment or electronic equipment, for preventing malfunction caused by mutual interference by said unnecessary electromagnetic waves among internal circuits and also for preventing the adverse effect by external electromagnetic waves. More particularly, the present invention relates to an electromagnetic wave absorber comprising a composite magnetic material having a soft magnetic material powder dispersed in a binding material.
2. Description of the Related Art
Problems regarding EMC (electromagnetic compatibility), in which unnecessary electromagnetic waves leak from the circuits of communication equipment and electronic equipment or such electromagnetic waves cause the equipment to malfunction, are more likely to occur, as smaller, lighter, and more sophisticated electronic equipment is made and the packaging density of electronic components increases drastically.
In addition, as recent communication and digital technologies advance, the applied frequency becomes higher, so that a point of view that is different from conventional one is necessary to address EMC. Further, in mobile phones that have recently been spread rapidly, the possibility of the adverse effect of radiated electromagnetic waves on the human body is also pointed out.
For measures against the generation and leakage of unnecessary electromagnetic waves and the malfunction caused by electromagnetic interference, methods of providing a shield to the source of the unnecessary electromagnetic waves or inserting a choke coil or a filter in a transmission line are generally employed. Along with such methods, methods for providing, near electronic components and circuits, an electromagnetic wave absorber having a soft magnetic material powder dispersed in a binding material such as a rubber or polymer material are also proposed and put to practical use. This electromagnetic wave absorber is excellent in processability and packaging ability and is highly practicable (“Recent Technology and Application of New Electromagnetic Wave Absorber,” Mar. 10, 1999, CMC, pages 124 to 125, Japanese Unexamined Patent Application 11-87117, 11-354973, 2000-4097, and 2002-158488).
However, according to recent demands for smaller, lighter, and more sophisticated electronic equipment and for higher frequency as described above, especially thin, high-performance members addressing high frequency are also required as members for measures against unnecessary electromagnetic waves.
The electromagnetic wave absorption performance of an electromagnetic wave absorber is determined by the product of the imaginary part relative permeability μ″ and the material thickness. Therefore, when the thickness is tried to be reduced, the value of the imaginary part relative permeability μ″ should be increased. Specifically, a thickness of 1.0 mm or less is required recently, and in this case, the necessary imaginary part relative permeability μ″ is required to be a value of 10 or more.
Conventionally, electromagnetic wave absorbers comprising a powder of an alloy that is widely known as a soft magnetic material, such as Fe—Al—Si (Sendust), Fe—Si (silicon steel), or Ni—Fe (Permalloy), combined with a binding material, such as a rubber or polymer material, are proposed in Japanese Unexamined Patent Application 11-87117, 11-354973, 2002-158488, and the like. By making flat the shape of this magnetic material powder, the imaginary part relative permeability μ″ is increased at a high frequency range.
The imaginary part relative permeability μ″ is changed by frequency, but with the above conventional flat alloy powders, the imaginary part relative permeability μ″ shows a maximum value at 1 GHz or less, and it decreases monotonously at a frequency range of 1 GHz or more in many cases. Therefore, the value of the imaginary part relative permeability μ″ is insufficient at a frequency range of 1 to 3 GHz, the application of which is recently promoted in portable phones and the like. In addition, with some of conventional flat alloy powders, in which the imaginary part relative permeability μ″ shows a maximum value at 1 GHz or more (Japanese Unexamined Patent Application 11-87117), the value of the imaginary part relative permeability μ″ itself is insufficient. Therefore, when the conventional flat alloy powders are used, the thickness of the electromagnetic wave absorber must be increased.
The imaginary part relative permeability μ″ of an electromagnetic wave absorber comprising a soft magnetic material powder and a binding material follows the logarithmic mixing rule. Therefore, for obtaining a high imaginary part relative permeability μ″, high volume fraction of the soft magnetic material powder has been adopted. However, if the volume fraction of the soft magnetic material powder is increased, the reflection of electromagnetic waves increases, which is not adequate for coping the problem of cross talk and the like inside electronic equipment.
Accordingly, the purpose of the present invention are to provide an electromagnetic wave absorber comprising a soft magnetic material powder and a binding material, which is thinner than conventional ones, has a high absorption performance for electromagnetic waves of 1 to 3 GHz, the application of which is promoted in electronic equipment and communication equipment, especially portable phones and the like, and, more preferably, has a small electromagnetic wave reflectance.
The inventors have studied for increasing the imaginary part relative permeability μ″ of the electromagnetic wave absorber at 1 to 3 GHz without increasing the reflectance. As a result, it has been found that by using a flat powder of a soft magnetic material comprising an alloy of Ni-30-60% Fe, the imaginary part relative permeability μ″ shows a maximum value at a frequency range of 1 to 3 GHz, and a large value (for example, 10 or more) can be obtained. Also, it has been found that by limiting the average diameter of the flat powder of the soft magnetic material in this case, a small electromagnetic wave reflectance can be obtained. The present invention is completed based on these findings.